Whole house air leakage

I have reviewed, tested, studied, asked, and analyzed the many facets of whole house air leakage. I am still somewhat unsure what constitutes a respectable level of air leakage to a home. Newer homes built to higher standards are not a problem but when you get into older homes with varying degrees of air leakage depending on when they were built and by whom presents some interesting situations. The recommended rates of leakage on older homes do not present the same situation as new homes as there are many areas of the home which are not accessible and to properly be able to reduce the leakage to ASHRAE guidelines would mean in many cases a major teardown and reconstruction to be able to seal the leakage to the "standards". In addition, many of these homes were built on the premise that the leakage would keep the wood and structure dry, minimizing those types of problems.

If you have suggestions, references and guidelines, please let me know. Thank you for your input.

Replies to This Discussion

I've tackled quite a few air sealing projects on homes built in the 1800's in the Colorado Rockies. They can be quite a challenge to air seal, even at the 10% required by our utility for rebate funds. If the crawlspace is accessible, I've found that spray foaming the rim joists can achieve a 10% reduction and not create moisture problems, especially when paired with a small crawlspace fan. If the rim joist isn't accessible and or/attic aren't accessible (many of our homes are built on something called a "mud-sill" another story altogether), I try to focus on air sealing inside the home which is often less effective. The important thing to keep in mind with regard to moisture is point-source control - make sure vent fans are installed in the bathrooms and kitchen and that the homeowner is educated about the importance of using them. Also avoid caulking the underside of lapped wood siding as that can trap moisture. Vapor permeable paint inside, no vapor barrier, and cellulose insulation inside the walls seems to do the trick in our climate.

Thank you for your input. I am curious if after you complete sealing the rim joists or other areas whether you performed any moisture tests after the sealing to determine the moisture content. Our area does not normally have moisture issues (20-25%) but I like to follow up when condensation and humidity is prevalent, usually in the colder weather and test a customers home for humidity in the home and evidence of mold or mildew buildup.

I like your idea of a crawlspace fan which will insure proper ventilation.

Roy, I have not tested for humidity post-retrofit, but that isn't a bad idea. I do know the crawlspace fan works to dry out the crawlspace; we had a client with a flooded basement and installed a vent fan which dried it right up in a few days.

Ideally the crawler also gets a sealed vapor barrier on the floor to prevent ground moisture and soil gases from entering the home. This isn't always possible given the constraints of some crawlspaces.

Again, I also stress point source moisture management with my clients, and that sometimes includes installing a vent fan or connecting it properly (sometimes people just dump their vent fans, including the drier fan, into the attic or crawlspace).

One other thing I forgot to mention is the importance of conducting CAZ testing post-retrofit to ensure that the dynamics effecting atmospherically-vented combustion haven't changed. I've never seen an issue but that doesn't mean it can't occur.

If your goal is to save energy, the answer is always "get it as tight as possible" If your goal is to save energy and assure indoor air quality, the answer is "build tight and ventilate right."

If you have to keep bean-counters happy, there are any number of good decision tools to help balance cost of work against value of energy saved. (See Appendix A-12 of Krigger's "Residential Energy" book for a simple but effective tool.) Few of these tools really take into account just how much energy costs are going to increase, but they are good starts.

If you don't have a lot of experience costing this out, remember that air sealing supplies are cheap. We expect to spend $30-$40 on labor and $7-$10 on caulk and foam to get 100 CFM50 of reduction in one hour. "Your results may vary" A LOT, but that 4:1 ratio is pretty common. And the cost is not that far out of whack, after you have already chased down the big holes.

That said, trying to apply new construction standards to retrofit just won't work. It makes a lot more sense to look at how much you can improve what you've got. A house that's already pretty tight is just not going to improve as much as one where your first job is to install entire window panes.

You can always look at percent improvement as a targeting tool. As one reply already noted, a single well-chosen measure can get a 10% reduction in many homes. In my world (low-income weatherization) an experienced crew that doesn't get a 10% reduction is having a bad day. That is, you are just about as likely to take a house from 2,000 CFM50 to 1,800 as to get a 5,000 CFM50 house to 4,500.

The average in our program is a 20-25% reduction -- the big, old 5,000 CFM50 barns go down 40% and the 40-year-old single-story ranch homes go down 10%. Ultimately, that 2,000 CFM50 ranch is probably never going to be taken under 1,500. But that 5,000 CFM50 beater may well be improved by 40% or more before you are done, especially if the walls get dense-packed.

I have been trying for a while to come up with a probability curve, but it just doesn't fly. Existing housing stock varies too much, even just within my own state. But, IMHO, good rules of thumb, though (at least for heating-dominated housing stock) are...

Any crew should be able to improve almost any existing house by 10%.

A good crew should be able to improve ALMOST any existing house by 20%, unless it starts under around 1,500 CFM50.

It's hard to improve the typical house beyond about 30% unless you dense pack all the walls.

I have seen stellar crews air seal and dense-pack big, old barns starting at 5,000 CFM and walk away with more than 50% improvement. It isn't easy, but it can be done...

For example, Figure 7. shows the blower door readings before and after weatherization, indicating average infiltration reduction. If I'm reading it right, it shows a statewide average pre-Weatherization blower door reading for homes in the program is 3000 CFM50. Weatherization takes the final blower door test down from that by an average of 1000 CFM50. But as Don says, there's a lot of variation depending mostly on where you start out in terms of leakiness.

Old homes are not great candidates for ASHRAE standards. It can be achieved with risk. Unless the energy rater understands all the hidden problems associated with the old home in question, air sealing can present some long term health issues with the occupants. Use extreme caution, because your liability does not end at air sealing and passing the blower door test when someone gets sick. Here in Connecticut people are air sealing homes without treating the radon issue or even properly ventilating the home. Reason = budget of the client. I'm not so certain this will work for the rater when people get sick. Be safe, work safe and check your insurance policy for this form of risk before tampering with an old home.

Air sealing can also provide an opportunity to reduce contamination from dirty attics and crawlspaces though, but you are right that ventilation is key which is why I think crawlspace fans are a good idea.

I have been thinking quite a bit about this topic recently, and the more I think about it the less convinced I am that we are on the right track.

First - blower door tests DO NOT give an indication of air infiltration; they give a slight indication of the COMBINATION of air infiltration and exfiltration. This is especially true if there are natural ventilation combustion devices, non-sealed fireplaces, non-Energy Star kitchen/bath fans, and clothes dryers. The blower door test depressurizes the home and draws air in through all these (normally exfiltration) devices and actually masks the amount of air infiltration as the combined exfiltration rate (in older homes especially) is far greater than the infiltration rate and provides a much greater "path of least resistance". It would be a far more useful test protocol to block off all these types of exfiltration devices prior to the blower door test to see just how much infiltration was provided, and where the infiltration actually occurs. Then air flow matching efforts could be employed (e.g., to ensure that the combustion zone was receiving as much combustion air as it was venting, to provide the same infiltration/supply to each separate living space, etc.). Current blower door test protocols really do not provide much usable information on specific remediations (as often noted by the Building Science Corp. heads).

+1 to everything you said. Something to add is that it depends entirely on the metric you use. If you use ACH50 which is becoming the norm where the 2012 IECC is used, you must seal every square foot of envelope on a 1200 ft2 house on a slab to be TWICE as tight as every square foot of a 3200 ft2 on a basement to achieve the same number.

I'm not sure I follow. Is it because the 2012 IECC does not include the basement area as conditioned space, or that slab houses are viewed as being "leakier" than houses with double the flooring footprint and having a basement, or that homes of a smaller volume are penalized? I have yet to see a basement that was not closely coupled to the "traditional" conditioned space nor have I seen any efforts to specifically de-couple basements from the floors above (or the stairways leading there). I would think slab houses that are constructed with the latest building science techniques would be every bit as tight as non-slab construction. Now if you have a greater interior volume, the acceptable "leakiness" limit will also be of a larger volume for the same ACH50.

Do the math on that one - any volume based measure favors big houses because the volume is not what is leaking - the surface area is. A small house has more surface area per ft3 than a big house, so gets penalized. I have measured several 4,000 ft2 houses with zero air sealing at 2 ACH50 and many 1,000 ft2 townhouses that have the crap sealed out of them at 4 ACH50.

And check the definition of "conditioned space" in both the '09 and '12 IECC.